1. Field of the Invention
The present invention relates to a calibration standard in the form of a coaxial coupling comprising a coaxial airline, in particular a coaxial plug or a coaxial socket for connecting to a coaxial coupling of a device to be calibrated. More specifically, the present invention relates to a measuring port of a vectorial network analyzer (VNA), wherein a coaxial coupling comprises an inner conductor part and an outer conductor part arranged coaxially therewith and a short-circuit connection provided between the inner conductor part and the outer conductor part.
2. Description of Related Art
The high level of accuracy of vectorial network analyzers (VNA) is based thereon that the network analyzer is calibrated before the actual measurement of the value and phase of the complex reflection coefficient by connecting calibration standards to its measuring ports. There now exist several different calibration methods. Most of the calibration methods use “open”, “short” and “match” calibration standards for system calibration. By connecting these calibration standards to the measuring ports of the network analyzer, the errors arising in the network analyzer that lead to a deviation of the measurement values from the true value can be determined and, during subsequent object measurement, used for computational error correction. This is disclosed, for example, in DE 39 12 795 A1. However, these previously usual calibration methods are still not sufficiently accurate. In order to determine the remaining uncertainty in the directivity and measuring port matching, it is proposed in EA guidelines that a precision coaxial airline which has a defined incorrect termination or short-circuit at its output should be connected to the measuring port of the previously system-calibrated network analyzer to be tested and the reflection coefficients at the input of this airline should be measured at a series of measuring points within a defined frequency range of the network analyzer (EA-10/12, EA Guidelines on the Evaluation of Vector Network Analyzers (VNA), European Co-operation for Accreditation, May 2000). According to these guidelines, it is only the ripple amplitude of the oscillations superimposed on the value of the reflection coefficients that is evaluated, although by way of simplification, it is assumed that this ripple amplitude is approximately identical with the effective source port match, although this only applies when the effective directivity is ignored. This known verification regulation, making use of a precision airline is therefore relatively inaccurate and does not enable a precise estimation of the measuring uncertainty that is to be expected, let alone any subsequent correction of the error correction terms for source port matching.
DE 102 11 334 A1 discloses a method for measuring the effective directivity and/or the effective source port match of a measuring port of a system-calibrated vectorial network analyzer, whereby a precision airline short-circuited at the output is connected and the complex reflection coefficient is measured at the input of this precision airline with a series of measuring points within a predefined frequency range. The effective directivity of the series of measured complex reflection coefficients is subjected to a discrete Fourier transform and the base band is filtered out of the resultant spectrum. By means of an inverse Fourier transform, the series of effective directivity values is obtained.
The calibration standard required for measurements of this type in the form of the short-circuited precision airline is matched to the corresponding coaxial coupling on the device to be calibrated. Coaxial couplings are usually designed such that, on plugging together, they abut one another with the respective outer conductors before the inner conductors abut one another at their end faces, in order to avoid damage within the coaxial coupling. The contact plane between the mutually abutting outer conductors is designated the reference plane. In the case of coaxial couplings made precisely to measure, due to component tolerances, it would be possible for the inner conductor of plug and socket to abut one another earlier than the outer conductors. If the plug and socket are then pushed further together with suitable force in order to permit the outer conductors also to contact one another at their end faces, the coaxial coupling suffers damage in the region of the respective inner conductors. It is therefore usual to design the plug and the socket with different separations of the end face of the inner conductor contact from the outer conductor contact plane (reference plane), so that when the outer conductors contact one another, a separation (offset) remains between the end faces of the inner conductors of the plug and the socket. An offset of this type is, for example, 2/100 mm to 3/100 mm. This protects the plug connection against damage due to forces between the inner conductors.
However, in the aforementioned calibration method, this offset affects the measurement result in an undesirably negative manner or falsifies them in an unforeseeable way and therefore diminishes the accuracy. It is therefore desirable to have a calibration standard available wherein the offset of the coaxial plug connection between the device to be calibrated and the plugged-in calibration standard can be adjusted to zero. For this purpose, the inner conductor of the calibration standard would have to be designed displaceable relative to the outer conductor of the calibration standard. Particularly in the case of the calibration standard in the form of the short-circuited precision airline, however, the short-circuit connection between the inner conductor and the outer conductor is a potential error source if it is not carried out exactly. Therefore, in the calibration standard of the “short” type, this connection is designed to be fixed and not detachable.